Science and the development of vaccines have helped eradicate many childhood diseases, but one continues to be passed down from generation to generation with no known medical cure: cooties. While temporary relief can come from a shot accompanied by the phrase, “Circle, circle, dot, dot, now you’ve got the cootie shot,” future flareups are always possible, though at least it’s a contagion that one eventually grows out of contracting. I’m obviously being a bit playful here, but one does have to give screenwriters Ian Brennan (“Glee,” “Scream Queens“) andLeigh Whannell (“Saw,” “Insidious“) some due for the inspiration of using cooties as a launchpad for a horror/comedy. However, it’s a shame that so much potential doesn’t have much in the way of follow-through.

Elijah Wood leads “Cooties” as Clint, an aspiring writer struggling with the first draft of his novel, who lives at home, and turns to teaching to make a few extra dollars. His latest gig finds him substituting at his old elementary school, where he meets the roster of oddball personalities he will soon have to fight alongside to stay alive: pickup truck driving, alpha male, gym teacher Wade (Rainn Wilson); his endlessly perky (“You know how I beat the terrorists after 9/11? With my positive attitude”) girlfriend Lucy (Alison Pill); socially inept and awkward Doug (Leigh Whannell); closeted Tracy (Jack McBrayer); and buttoned-up, right-wing nutjob Rebekkah (Nasim Pedrad). Just as in any school, these strident and ill-fitting teachers somehow manage to balance each other out, forming a common unit, and the film manages to convey that quite easily. But these characters perhaps never thought they’d be forced to bond quite like this.

Impressively, “Cooties” doesn’t waste a moment in getting right down to action. The opening credits show a batch of chicken nuggets, made from spoiled meat, being processed, packaged, and delivered to the school, where one particular tender filled with gross ooze, gets a healthy bite from a young girl. As the movie begins, she arrives at school, in Clint’s class no less, where looking already half zombiefied, she lashes out violently at a bully, biting off a piece of his cheek (with plenty of blood and entrails, the movie earns its R-rating). The young boy quickly becomes bloodthirsty and brain dead too, and by recess, so are the rest of his classmates, with the teachers barricading themselves indoors from the mayhem, figuring out what to do next. Directors Jonathan Milott and Cary Murnion establish the film’s premise with panache and some big laughs, but keeping up the film’s witty, breakneck pace proves hard to sustain.

Delivered with a knowing awareness of horror tropes, but without feeling the need for indulgent winking at the audience, the film runs on a fresh and vibrant vibe in the early stages. But when the movie essentially turns into a teachers vs. kid zombies movie, Brennan and Whannell’s script becomes dully conventional. Whatever distinguishing features that would’ve made the film stand out from the zom-com pack gradually get sanded away, and attempts to broaden the film’s genre palette with a couple of dramatic scenes, and exposition about the rules for these zombies, suck the life out of the movie (pun intended). Perhaps more egregious is the clunky insertion of a new character into the script at a critical juncture to get the ensemble cast out of a corner they were written into, and propels “Cooties” from the second to third act. It feels like a cheat and the film never quite recovers. And speaking of unwieldy shortcuts, there’s also Jorge Garcia‘s Rick, the school’s security guard, who literally sits in a van for almost the entire movie, until he too becomes handy when the script requires.

Indeed, the latter third of the film betrays the emptiness at the core of “Cooties.” One particular scene has the cast of characters walking by a movie theater, and the posters on display are for “Cooties” itself, and last year’s horror hit, “A Girl Walks Home Alone At Night,” both films produced by Wood. It’s an odd moment in a movie that otherwise makes clear references to “Christine,” “Commando,” and “Mad Max Beyond Thunderdome,” and it’s hard to know what it means. But perhaps like those disparate nods, it actually doesn’t mean anything. Certainly, the biggest shame of “Cooties” is that given its pointed, racially homogenized setting, jabs at conservatism, and mentions of over-medicated children and absentee parents, the film actually has very little to articulate about any of those subjects.

By the film’s finale, it fully commits to being just another zombie killing machine, with whatever charms of the early stages mostly evaporated. The engaging opening third of “Cooties” is enough to make the rest of the 96-minute film a mildly amusing diversion, but as the minutes roll by, you’ll wish the brains of the film had remained intact. [C]